The rates of growth and the formation of products or metabolites by aerobic microorganisms are often regulated by the availability of dissolved oxygen in the culture medium. While a low level of dissolved oxygen inhibits the growth or production of certain microbial metabolites, concentration of oxygen above the critical level may not affect activity. For this reason, it is desirable to maintain high oxygen supply during the cultivation of aerobic organisms.
In stirred fermentors, aeration is carried out by injection or sparging of air into the vessel. In this process, a high level of dissolved oxygen is easily achieved. For media contained in flasks or tubes, aeration is essentially by simple gas-liquid contact: oxygen is transferred into the liquid culture from the gas head space. Thus, the surface area of the liquid medium is critical in the transfer process. The larger the surface area exposed, the better is the aeration for the medium. Exposure of the larger surface area is carried out by shaking the flask; this creates a vortex, exposing more liquid surface area to the head space.
Other methods devised to increase aeration include indentation of the flask or provision of baffles at the bottom of the flask. Thus, when the flask is shaken, the baffles break up the vortex and the liquid media is splashed, creating a considerable increase in surface area beyond that afforded by the vortex.
An increase in surface area of the medium inside the flask does not necessarily translate, however, to high aeration. The supply of air inside the vessel is another limiting factor. The length of the neck of the flask and the type of closure can greatly affect the flow of air to the surface of the liquid. Flasks or tubes used in the culturing of microorganisms are traditionally sealed with cotton plugs to prevent contamination. Cotton plugs, however, are not good closures in regard to air permeability; they offer considerable resistance to gas transfer. Consequently, cotton plugs allow only a low level of oxygen diffusing into the vessel and poor escape of carbon dioxide resulting in its build up inside the vessel.
Several improvements in closures have been introduced since the cotton plug, including loose-fitting stainless steel and plastic closures, gauze, foam pads, and silicone sponge plugs. The loose-fitting closures allow air to slip or diffuse from under the cap over the lip of the flask or tube, while cotton or gauze closures allow air to enter the vessel from the top through the closure. Entrance of air by these methods is poor; moreover, the air has to traverse the length of the neck of the vessel. For these reasons, the conventional flask systems are inferior to the air-sparged fermentor system in achieving high levels of dissolved oxygen in the medium. Despite the need for improved small-scale systems and technology, the same has not been available.